Doors, bumpers, and numerous other automobile members, members for home electrical appliances, building materials, etc. are produced by press forming steel sheet. In recent years, demands have arisen for reducing the weight of these members. To meet these demands, high strength steel materials are being used to reduce the thickness of the steel materials etc.
However, along with the higher strength of steel sheet, stricter control has become necessary to secure the product form by press forming. One of the important items in this control is the deformation when an elastically deformed part of steel sheet elastically recovers driven by the residual stress occurring in the steel sheet at the time of press forming, that is, the springback.
In particular, recently, to reduce the work and cost in development of automobiles etc., the trend has been to start the mechanical design stage of studying the method of forming the formed members at the same time as the conceptual design stage. Changes in conceptual design in the conceptual design stage cause changes in the formed members at the mechanical design stage, so the work and cost in the mechanical design stage of studying the method of forming the formed members become greater problems in the development process or development costs of automobiles etc.
FIG. 1 gives cross-sectional schematic views of a formed member showing the conventional measure against springback. (a) shows the cross-sectional form of a formed product, (b) shows the springback occurring in a formed product after cold pressing steel sheet by tooling of the same form as the formed product shown in (a), and (c) shows the cross-sectional form of tooling corrected envisioning springback. In this way, to obtain the formed product shown in (a), there is the measure of using the tooling “anticipating” springback as shown in (c) to obtain the desired formed product.
As a method of forming tooling anticipating springback in this way, the method of using the finite element method to analyze the residual stress of steel sheet at bottom dead center when pressed by the tooling and numerically analyzing tooling of the form of the deformation (spring forward) occurring due to the residual stress in an opposite direction to that residual stress so as to form tooling simply considering springback (“Japanese Patent Publication (A) No. 2003-33828” and “Mitsubishi Motors Technical Review (2006, No. 18, p. 126 to 131)”).
However, designing tooling completely considering springback by numerical analysis is a nonlinear problem and extremely difficult, so the proposed method forms tooling simply considering springback by the finite element method. Therefore, what measure becomes necessary when the allowable amount of springback cannot be met due to such tooling is difficult to analyze numerically, so no method of solution has been proposed up to now.
Therefore, what sort of measure to take to obtain the desired formed product when the allowable value of springback cannot be satisfied by tooling simply considering springback depends on the experience of the engineer. In the final analysis, trial-and-error tests by tooling obtained by the forming method and the actual steel sheet become necessary.
Further, a method has been proposed of reducing springback by making modifications to relieve residual stress not in the tooling form, but the form of the steel material or the formed product.
FIG. 2 gives perspective views illustrating a conventional method of finding a location becoming a cause of occurrence of deformation due to springback. (a) shows the form of a formed product, (b) shows the case of cutting out a part 1 of a product, (c) shows a case of forming a hole 2 in the product, and (d) shows the case of making slits 3 in part of the product. Such measures are devised, then the springback behavior is observed and measures tested out to reduce the springback.
However, while the measures against the location of occurrence of springback reduce the residual stress causing springback, cutting out parts, forming holes, etc. cause the rigidity of the member itself to fall, so the problem arises that springback ends up occurring due to slight residual stress. The fundamental cause has therefore not been found. Furthermore, this sort of measure in practice requires actual tests by test tooling and steel sheet, so the problem arises of an increase in work and cost in the mechanical design stage.